2010 Summer Student Programs Seismic features attribute before

2010 Summer Student Programs Seismic features attribute before and after the Chi-Chi earthquake: Temporal recovery 集集地震前後區域地震特徵的變化 : 震後復原行為學生：林承恩(Cheng-en Lin) 指導老師：陳卉瑄(Kate Huihsuan Chen)

1999 Chi-Chi earthquake in Taiwan • The M 7. 6, 1999 Chi-Chi earthquake ruptured the Chelungpu thrust fault along the western foothills of Taiwan extending approximately 100 -kmlong with up to 10– 15 m of coseismic slip.

Sudden change in regional seismicity near the Chi-Chi source region • Before the Chi-Chi earthquake, the central-west Taiwan tend to be seismically inactive. • After the Chi-Chi earthquake, the biggest change in earthquake activity occurred in hanging wall of the Chelungpu fault (1), middle the Chelungpu fault (2), and the Chiayi area (3). 2 1 3

Questions • How can we explain the spatial distribution of sudden change in seismicity? - Calculation of static Coulomb stress for explaining the aftershock patterns. - If the increase in Coulomb stress change corresponds to aftershocks, then we can explain the post-Chi. Chi seismicity by static stress change induced by the Chi rupture. • How long it would take to recover fault strength to the pre-Chi. Chi level? - Calculation of temporal decay of aftershock rate. - Recovery of repeating events’ waveform characteristics.

Quantify the seismicity rate change Z-value: R 1 : mean seismicity rate in the overall period the time R 2 : mean seismicity rate in the window of interest σ1、 σ2 : the standard deviations of seismicity rate n 1 : the number of samples in the overall period the time n 2 : the number of samples in the window of interest positive Z-value decrease in seismicity rate negative Z-value increase in seismicity rate.

Can the static Coulomb stress transfer explain the seismicity change? Coulomb Failure Function Failure initiates and spreads on the fault plane when the Coulomb Failure Function exceeds a specific value: Pore fluid pressure The shear stress on the failure plane (left: +) Coefficient of friction The normal stress on the failure plane (extension: +)

Coulomb stress change model Stress increase promote, and decrease inhibit, fault failure (e. g. , Harris, 1998; Stein, 1999; King and Cocco, 2000) Co-seismic slip model (Hsu et al. , 2009) The stress changes computed from coseismic displacement

Increase of stress change vs. aftershock distribution • Stress change model (background red to blue color) • Earthquakes observation (red circles) • Stress change increase zones correlate with the aftershock in 3 months of the Chi-Chi earthquake The stress changes computed from coseismic displacement

How long does it take to recover fault strength to the pre-Chi. Chi level? Measuring aftershock duration Aftershock duration The time period when the earthquake rate goes back to the pre-Chi level define the aftershock duration: 16. 4 yr.

Repeating Earthquakes are a useful tool for measuring temporal changes of wave propagation in the Earth’s crust • Source and site effects are common to all events in a sequence

Repeating sequences found near the Chi-Chi source region so far • Grey circles: background earthquakes • Circle in color: five different repeating earthquake sequences • Given the uncertainty of earthquake location from CWB earthquake catalog, the repeating events in each sequence still clustered in space.

• Collection of 6 events recorded by EHY station for one sequence. Each trace is a superposition with the reference event, showing the high waveform similarity.

Detailed change in waveform characteristics by moving window analysis • (A) Nine sub-windows for waveform similarity calculation and each scan 3 seconds. • (B) Seismograms from reference event (red) and event 3 (black) in sequence recorded by EHY station. Reference event occurred on 27 December 1998 and event 3 was 1 month after the reference event. • (C) Waveform cross-correlation coefficient of this even-pair at different subwindows, showing large decrease at the S-coda.

Waveform similarity drops for the post-Chi. Chi event (observation at different stations) With the subtle change in waveform similarity for the post-Chi repeating events, we show that 1) The medium property change is widely distributed, not only localized near the fault rupture. 2) the healing of subsurface damage zone may not fully complete 1 year after the Chi-Chi earthquake. We need more updated events to see how long it would take for the full recovery to occur.

Conclusions and future works • The spatial distribution of sudden change in seismicity is due to the stress adjustment induced by Ch-Chi coseismic rupture. • The background seismicity level return to the pre-Chi level in 16. 4 years. • With common source and site effects to all repeating events in a sequence, variations in wave propagation path can be monitored by the changes in the character of seismic waveforms. However, the repeating earthquake identification takes more than 2 months, and the computation will be more complete in the following months.

References • • Stefan Baisch & Gotz H. R. Bokelmann 2001: Seismic waveform attributes before and after the Loma Prieta earthquake: Scattering change near the earthquake and temporal recovery Chung-Han Chan& Ross S. Stein 2009: Stress evolution following the 1999 Chi-Chi, Taiwan, earthquake: consequences for afterslip, relaxation, aftershocks and departures from Omori decay Kuo-Fong Ma and Chung-Han Chan 2005: Response of seismicity to Coulomb stress triggers and shadows of the 1999 Mw = 7. 6 Chi-Chi, Taiwan, earthquake Ya-Ju Hsu, Jean-Philippe Avouac, Shui-Beih Yu, Chien-Hsin Chang, Yih. Min Wu, and Jochen Woessner 2009: Spatio-temporal Slip, and Stress Level on the Faults within the Western Foothills of Taiwan: Implications for Fault Frictional Properties

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